Pathophysiology
PAH is a disease which affects small pulmonary arteries. It is characterized by vascular obstruction leading to progressive increase in vascular resistance. This increases right ventricular afterload and consequently results in right ventricular failure. Intima and media proliferation and its consequent pulmonary vascular obstruction are considered to be the key element in the pathogenesis of PAH. Vasoconstriction, vascular remodeling and thrombosis are factors that increase pulmonary vascular resistance in PAH [107,137]. These processes involve a multitude of cellular and molecular elements (Figure 1).
Figure 1  Pathophysiology of PAH. The pulmonary vascular remodeling responsible for PAH is the consequence of closely intertwined predisposing and acquired factors. Thoses pathological elements affect all three layers of precapillary pulmonary arteries leading to intimal hyperplasia, medial thickening and adventitial remodeling/fibrosis. Intra- but also extra-pulmonary cells, such as inflammatory and progenitor cells, are suspected to play a role in this remodeling. This increases right ventricular afterload and consequently results in right ventricular failure.

Cellular factors
Proliferation of smooth muscular cells in the small peripheral pulmonary arteries is a common characteristic in all forms of PAH. In hypoxic models, fibroblasts of the adventitia migrate to the media and intima, where proliferation and production of matrix proteins are observed [138]. Neovascularization, mainly of the adventitia, occurs concomitantly to the thickening of the vascular walls [139].
In response to certain stimuli, endothelial cells abnormally proliferate to form plexiform lesions in several forms of PAH. Plexiform lesions consist of endothelial cells, matrix proteins and fibroblasts and obliterate the vascular lumen [140]. The stimuli for endothelial proliferation is still unknown but several factors have been incriminated such as hypoxia, inflammation, shear stress, drugs, viral infections and genetic susceptibility. Extrapulmonary cells may also participate in the vascular remodeling responsible for PAH. Indeed, fibrocytes and c-kit + cells are mobilized from the bone marrow, and may differentiate into vascular cells and/or produce pro-angiogenic factors to participate in the pathogenesis of PAH [141,142]. The CXCL12/CXCR4 axis may play an important role in the pulmonary recruitment of these circulating progenitors and can be therapeutically targeted [143].
Inflammatory mechanisms seems to play an important role in certain forms of PAH such as PAH associated with auto immune diseases or HIV infection [144]. In fact, in severe cases of PAH associated with systemic lupus erythematosus disease, some patients improved both clinically and hemodynamically with administrated immunosuppressant treatment. Thirty to 40% of patients with PAH have circulating auto-antibodies and elevated plasma concentrations of pro-inflammatory cytokines such as interleukin 1 (IL-1) and interleukin-6 (IL-6), and chemokines such as fractalkine and MCP-1 [145,146]. Inflammatory cells, such as lymphocytes B and T, macrophages, mastocytes and dendritic cells, can also be found in plexiform lesions of severe PAH [147,148]. Chemokines, like RANTES and fractalkine are also overly expressed in the pulmonary vascular endothelium of PAH patients [145].
Thrombosis and platelet dysfunction can be important in the development of PAH. Abnormalities of thrombosis, endothelial cells or platelets can generate or aggravate thrombosis in situ. Elevated plasma concentrations of D-dimers and fibrinopeptides A and B, in certain patients with PAH, are the proof of an abnormal intravascular coagulation process. Elevated plasma concentrations of von-Willebrand factor and plasminogen activator inhibitor type 1 also reflect endothelial dysfunction in PAH. It has been demonstrated that shear stress creates pro-thrombotic vascular lesions in PAH that may lead to thrombosis in situ. But platelet function is not limited to coagulation. In response to certain stimuli, platelets can produce prothrombotic, vasoactive or mitogenic factors, such as thromboxane A2 (TXA2), platelet-derived growth factor (PDGF), serotonin (5-hydroxytryptamine, 5-HT), transforming growth factor beta (TGF-β) and vascular endothelial growth factor (VEGF) that participate in vasoconstriction and vascular remodeling [149,150].

Autoimmunity and PAH
The self-tolerance is controlled in the periphery by a particular population of T-lymphocytes called regulatory T-lymphocytes (Treg). The breakdown of self-tolerance can lead to the development of an autoimmune response (i.e. directed against self antigens) that can finally give rise to an autoimmune disease. Huertas et al. [151] showed that circulating Treg number was comparable in idiopathic PAH and SSc-PAH patients. However the percentage of those expressing leptin receptors was higher in idiopathic PAH and SSc-PAH as compared to controls, and their function was reduced in idiopathic PAH and SSc-PAH patients as compared to controls in a leptin-dependent manner [151]. Work on chronic inflammatory disorders and autoimmune diseases suggest that pathogenic antibodies and T cells may be generated locally, in the targeted organ, in highly organized ectopic lymphoid follicles commonly called tertiary lymphoid tissues. Recently, Perros et al. [152] described the presence of highly organized perivascular follicles in idiopathic PAH lungs arguing for specific immune-adaptive mechanisms in the pathophysiology of the disease. One can propose that deregulated and unresolved pulmonary inflammation on the background of a genetic predisposition, could result in persisting vascular remodelling leading to PAH. An initial acute inflammation that is normally expected to resolve with return to homeostasis, could conduct the production of auto-antibodies against vascular wall components, and would shift to chronic persisting and chronic inflammation, endothelial barrier breakdown, infiltration by immune cells, local and chronic autoimmunity, and vascular remodeling culminating in PAH.

Molecular factors
Many authors consider pulmonary vasoconstriction as an early event in the process of PAH. Vasoconstriction has been associated with an abnormal function or expression of potassium channels and with endothelial dysfunction [107]. Endothelial dysfunction results in a decreased production of vasodilators such as nitric oxide (NO) and prostacyclin and an increased production of vasoconstrictors such as endothelin-1 [153].
Prostacyclin (prostaglandin I2) is a potent pulmonary vasodilator that acts via the cyclic adenosine monophosphate (cAMP) pathway. It inhibits the proliferation of smooth muscle cells and decreases platelet aggregation. Production of prostacyclin is reduced in endothelial cells of patients with PAH [154]. PAH therapy based on prostacyclin and its derivates have proven efficacy both hemodynamically and in clinical trials. NO is also a pulmonary vasodilator which acts via the cyclic guanosine monophosphate (cGMP) pathway. To increase pulmonary vasodilatation dependant on NO, a recent therapeutic strategy has targeted type 5 phosphodiesterase which degrades cGMP. Sildenafil or tadalafil, type 5 phosphodiesterase inhibitors, have proven their efficacy in patients with PAH [155]. Vasoactive intestinal peptide (VIP) is a neurotransmitter that has systemic and pulmonary vasodilator properties. It also inhibits smooth cell proliferation and decreases platelet aggregation and acts via the activation of the cAMP and cGMP systems [156]. Low plasmatic concentrations of VIP have been measured in pulmonary arteries of patients with PAH.
Endothelin-1 (ET-1) is an endothelially-derived peptide that has two receptor subtypes, designated as endothelin A (ETRA) and endothelin B (ETB), located on smooth muscle cells of pulmonary arteries. By ligating the ETRA, ET-1 intracellular calcium concentrations increase and activates the protein kinase C pathway [157]. ET-1 is a potent pulmonary vasoconstrictor and stimulates mitosis of arterial smooth muscle cells, thus contributing to pulmonary vascular remodeling. Pulmonary and plasma levels of ET-1 are elevated in human PAH and in experimental animal models of PAH [158]. The therapeutic efficacy of endothelin receptor antagonists (Ambrisentan, Bosentan) has been demonstrated in clinical trials in the pathophysiology of PAH.
In hypoxic models of PAH, hypoxia inhibits one or several voltage dependant potassium channels of the pulmonary arterial smooth muscle cells. This leads to membrane depolarization and opening of voltage dependant calcium channels with a subsequent increase of the intracellular calcium concentration and cellular contraction. Certain potassium channels are under expressed in pulmonary artery smooth muscle cells of patients with PAH [159,160]. It is still unknown whether abnormalities of the potassium channels are acquired or genetic. However, it has been demonstrated that anorexigens, such as dexfenfluramine and aminorex, directly inhibit certain potassium channel subtypes [161]. Certain medications such as dichloroacetate and sildenafil increase the expression and function of potassium channels.
In PAH, plasmatic concentrations of serotonin (5-hydroxytryptamine, 5-HT) are elevated [150]. An association between anorexigens and serotonin was established in the 1960s. Aminorex and fenfluramin both increase plasmatic levels of serotonin. The variability of the expression and activity of the transporter of 5-HT (5-HTT) contributes to pulmonary vascular remodeling in human and experimental models of PAH [162]. Some studies have shown that the serotonin selective reuptake inhibitor fluoxetin prevents the development of PAH in mice [163]. Some 5-HT receptor subtypes may also be implicated in the development of hypoxia induced PAH [164].
Rho proteins regulate fundamental cellular functions such as contraction, migration, proliferation and apoptosis. Several studies have implicated Rho protein A and Rho kinases in the vasoconstriction and vascular remodeling of PAH [165,166]. RhoA and Rho kinase activities are increased in idiopathic PAH, in association with enhanced RhoA serotonylation. Direct involvement of the 5-HTT/RhoA/Rho kinase signaling pathway in 5-HTT-mediated pulmonary artery-smooth muscle cell (PA-SMC) proliferation and platelet activation during PH progression identify RhoA/Rho kinase signaling as a promising target for new treatments against PH [167].
Hypoxia inducible factor-1 (HIF-1) is a transcription factor that principally regulates cellular adaptation to hypoxia but also regulates several genes implicated in angiogenesis, erythropoiesis, cellular metabolism and survival [168]. In experimental mice heterozygote for the gene coding for HIF-1 alpha, hypoxia induced right ventricular hypertrophy, right ventricular pressure and medial thickening of pulmonary arterioles are reduced [169]. In immunohistological analysis of human plexiform lesions of patients with severe PAH, there was an overexpression of HIF-1 alpha in proliferating endothelial cells [170].
In conclusion, the pathophysiology of PAH is heterogeneous and multifactorial. The genetic mutations found in familial PAH and in a proportion of sporadic PAH are neither necessary nor sufficient for the development of PAH. Therefore, the current hypothesis is that of a genetic predisposition for PAH followed by a superimposed environmental factor (infection, inflammation, autoimmunity). Our understanding of the underlying pathophysiological mechanisms of PAH has lead to the development of new treatments such as prostacyclin analogues, endothelin receptor antagonists and type 5 phosphodiesterase inhibitors. However, future progress is still necessary in order to discover new pathophysiological pathways and to develop new therapeutic strategies in PAH.